A protection device of above mentioned general type is, in particular, preferably used for knee airbags which have a low-lying installation position such that, during inflation, the knee airbag has to be placed from below upwards in front of the vehicle structure. The deploying airbag therefore has to be positioned in front of the instrument panel or the glove compartment from the foot well.
DE 10 2006 040 177 A1 describes a subassembly with an instrument panel for motor vehicles and a knee airbag which, in the rest state, is accommodated in a housing. Upon activation, the knee airbag is inflated between a lower section of the instrument panel and the area of the knee and shin bone of the occupant to be protected. In order to make production of vehicle versions with and without a knee airbag easier, the housing is located on or in a shaped part adjoining the lower section of the instrument panel. Due to the large volume of the knee airbag, the internal pressure of the airbag is relatively low in a first deployment phase, which results in poor positioning of the knee airbag.
DE 298 07 424 U1 describes a knee protection device for vehicle occupants, with an airbag which, in the inflated state, extends in front of the vehicle occupant's knees in the restraint situation, bears against the knees in the restraint situation and is intended to substantially prevent the knees from shifting forwards. The knee protection device is intended to be distinguished by a high and specific restraining force in the region of the knees. This is achieved by means of catch straps (or tethers) which connect mutually opposite sections of the airbag wall. The catch straps are fastened to the airbag wall and are arranged in such a manner that they oppose a change in shape of the airbag when the knees plunge into the airbag. The catch straps serve as partitions and have at least one overflow opening. The overflow opening acts like a throttle, thus resulting in an overall more stable structure of the airbag and of the effect of an external force. The partition is not fastened at the sides to the airbag wall, thus producing overflow channels at the sides such that, when the airbag is deployed, the partitions negatively effect the inflating speed.
DE 10 2006 013 287 A1 describes an airbag for a motor vehicle, with a flow passage via which two pressure chambers are connected to each other, wherein a flow medium flows in a pass-through direction from one pressure chamber into another pressure chamber through the flow passage. The flow passage comprises a means for protecting against backflow which permits the flow medium to flow in the pass-through direction while the flow of flow medium in a blocking direction opposed to the pass-through direction is substantially prevented.
U.S. Pat. No. 5,458,366 describes a knee cushion device with an airbag which is divided into individual compartments. A gas generator conducts inflating gas into a collecting chamber via which the inflating gas is conducted into the individual compartments of the airbag. The collecting chamber is connected to the individual compartments via a one-way valve in each case. The collecting chamber is arranged behind the individual compartments in the direction of travel and does not contribute to supporting the knee cushion.
DE 20 2006 005 872 U1 relates to a knee protection device for vehicle occupants, with an airbag and a gas generator which is connected to the airbag via an inlet opening, wherein the airbag has at least two chambers which, in the installed, inflated state, are essentially located one above the other and are connected to each other via at least one overflow opening. The inlet opening of the gas generator is assigned to the upper chamber, and the overflow opening is designed in such a manner that the gas overflows from the upper chamber into the lower chamber until a uniform pressure is reached with a certain time delay in order to maintain a greater pressure in the knee chamber for a longer period of time.
DE 10 2006 051 218 A1 describes a vehicle occupant restraint device with an airbag module which has a housing and an airbag which has a free outlet end at which said airbag moves into the fully deployed position thereof. At least one control element is provided, the control element acting on the airbag and forcing the airbag in the deployed state into an overall curved profile from the housing as far as the free outlet end. The control element may be designed as a catch strap which acts on the rear and on a front envelope part of the airbag, wherein the fastening points of the catch strap are offset with respect to one another in the direction of deployment of the airbag. That end of the catch strap which is not assigned to the airbag is fixed on a vehicle structure or on the airbag housing.
DE 603 053 74 T2 describes an occupant leg protection device with a main chamber which is inflated first by gas which has been introduced by a gas generator. An auxiliary chamber which is inflated by the gas which is conducted through the main chamber is arranged along a surface of the main chamber in the vicinity of the occupant or along the surface of the main chamber away from the occupant. The main chamber has a plurality of chambers between which separating lengths of fabric are arranged. Intermediate spaces through which the gas can flow in the individual chambers are provided on the side borders of the separating lengths of fabric.
Since the knee airbags are generally fitted below an instrument panel and are therefore located relatively far away from the extremities to be protected, they require a relatively large volume. After the gas generator has triggered and an airbag covering opened, the collapsed airbag is ejected at high speed. During the subsequent, first deployment phase, the internal pressure of the airbag drops rapidly, since more airbag volume is provided than the gas generator can deliver. Said relatively low internal pressure results in mechanical instability of the airbag during the positioning in the first deployment phase.
It is therefore the object of the present invention to provide a protection device which takes the abovementioned problems into account and, despite an unfavourable mounting positioning in the rest state, ensures rapid and stable deployment of the airbag.